There is no definitive known cause of ODD in adults, but there are often patterns traced back to experiences of children with ADHD, according to ADDitude. As many as 84 percent of children whose ADHD goes untreated or improperly treated for two or three years develop ODD. Adults with ODD may often get fired from jobs for frequent conflicts. Treatment options for adults include stimulants or other medications used for ADHD and anger management classes.